1. Field of the Invention
The present invention relates to an optical information apparatus for reproducing information from an information recording medium represented by optical discs or recording information to the information recording medium; an optical head device for reproducing or recording information in the optical information apparatus; and information equipment and a system using the same. The present invention also relates to a diffractive element used therein.
2. Description of the Related Art
Since a digital versatile disc (DVD) can record digital information at a recording density of about six times the compact disc (CD), the DVD is known as an optical disc that can record large volume data. Recently, optical discs of larger volume are being desired because of an increase in the amount of information to be recorded on the optical disc. In order to achieve a large volume optical disc, the recording density of the information must be increased by reducing the optical spot formed by the light irradiated onto the optical disc when recording information on the optical disc and reproducing information recorded on the optical disc. Specifically, the optical spot can be reduced by using laser light having a short wavelength and increasing the numerical apertures (NA) of an objective lens. The wavelength of 660 nm (red) is used for the wavelength of the light source, and the objective lens having a numerical aperture (NA) of 0.6 is used in the DVD. Furthermore, in BD, the light source having a wavelength of 405 nm is used, and the objective lens having NA of 0.85 is used in order to achieve a recording density of five times the recording density of the current DVD.
In the optical information apparatus for realizing recordation and reproduction at high density using the laser of short wavelength by blue laser, the usability of the device is further enhanced by providing a compatibility function with an existing optical disc, and cost performance is also enhanced.
DVD-R having a track pitch of 0.74 μm and DVD-RAM for performing recording on both land (L) and groove (G) at 1.3 μm coexist in the DVD using red light source. Thus, it is important to stably perform track control on the optical disc of different track pitches even for equipment dedicated to use with a DVD, and in addition, compatibility with CD and BD is also desired.
To perform track control, the track shift amount must be detected, and the tracking error signal must be detected. One of the frequently used tracking error signal detection methods is a differential push-pull (DPP) method. The differential push-pull method is disclosed in patent document 1 (Japanese Laid-Open Patent Publication No. 7-272303), and will be described briefly using the figure.
FIG. 19 shows a configuration of an optical head device according to the prior art. The light beam 210 radiated from a light source 201 is transmitted through a diffraction grating 204. The diffraction grating 204 produces a conjugate diffracted light. The transmitted main beam and the diffracted sub-beam are converted to parallel light by a collimator lens 203. An objective lens 205 converges the main beam and the pair of sub-beams, which are parallel light, on a recording surface of the optical disc 227. The light reflected from the optical disc follows the same optical path in the opposite direction and enters a light detector 266. The objective lens is moved in the tracking direction based on the tracking error signal obtained from the light detector 266 to perform track control. The configuration of the diffraction grating used in FIG. 19 is shown in schematic form in FIGS. 20A and 20B. FIG. 20A is a front view and FIG. 20B is a cross sectional view. The diffraction grating 204 is obtained by periodically forming concave-convex parts on a surface of a transparent base material such as glass, as shown in FIG. 20B. For simplicity, only the center line of a convex part is shown in the front view of FIG. 20A. In the present application, unless particularly stated, only the center line of the concave part will be shown in other figures for simplicity when showing the concave-convex shape in front view. When the light beam is transmitted through the diffraction grating 204, conjugate±one-dimensional diffracted light is generated, and converged on the recording surface of the optical disc 227 as main beam 211 and a pair of sub-beams 212, 213 by the objective lens 205 as shown in FIG. 21. The recordable optical disc such as DVD-RAM has concave-convex grooves on the recording surface thereof. The concave-convex is referred to as land (L) and groove (G). The diffraction grating 204 is rotation adjusted as in the arrow of FIG. 20A so that when the main beam 211 converges on a certain groove, the sub-beams 212, 213 converge on the adjacent lands. When the push-pull signal generated by diffraction at the disc groove is detected from the main beam and the sub-beams reflected and returned from the disc, the positive and negative signs of the push-pull signal become opposite for the main beam and the sub-beams. The tracking error signal of push-pull signal method is detected through differential calculation of the push-pull signal of the sub-beams and the push-pull signal of the main beam. In this method, the distance in the tracking (T) direction of the main beam and the sub-beams must be equal to the inter-center distance of the land and the groove, that is, half of the track pitch, and thus the amplitude of the tracking error signal lowers when applied to the optical disc of different track pitches such as DVD-RAM and DVD-R.
The diffraction grating developed to overcome such problem is shown in FIG. 22A (patent document 2: Japanese Laid-Open Patent Publication No. 9-81942). As shown in FIG. 22A, the diffraction grating 224 is divided in half by a dividing line parallel to the groove direction Y. Assuming the grating phase of the first diffraction grating region 2241 on the left side of the figure is the reference (0 degree), the grating phase of the second diffraction grating region 2242 on the right side of the figure is 180 degrees. The shift of the grating phase in the T direction is positive. The sub-beams are symmetrically divided in half as shown in FIG. 22B. The sub-beams 222 and 223 are arranged on the same groove as the main beam 221. In this case, the push-pull signals obtained from the sub-beams 222 and 223 have opposite signs from the main beam 221. Thus, the tracking error signal of the push-pull signal method is detected by the differential calculation of the push-pull signal of the sub-beams and the push-pull signal of the main beam. In this method, arrangement is made such that the distance in the tracking (T) direction of the main beam and the sub-beams becomes 0, and thus the amplitude of the tracking error signal does not lower from the difference in discs even when applied to the optical discs having different track pitches such as DVD-RAM and DVD-R. However, when the objective lens moves in the tracking (T) direction by track following, the amplitude of the tracking error signal at the destination lowers.
Furthermore, a method contrived to avoid lowering in amplitude of the tracking error signal by the movement of the objective lens is disclosed in patent document 3 (Japanese Laid-Open Patent Publication No. 2000-145915) and patent document 4 (Japanese Laid-Open Patent Publication No. 2006-4499). As shown in FIG. 23A, a third diffraction grating region 2343 having a phase of 90 degrees is arranged between the first diffraction grating 2341 having a phase of 0 degrees and the second diffraction grating region 2342 having a phase of 180 degrees. The amplitude of the signal in a state the objective lens is not moved by track following is lowered in advance, and the lowering in amplitude of the tracking error signal in a state the objective lens is moved is relatively avoided by arranging a different region in the middle.